CS 3516: Computer Networks

Transcription

1 Welcome to CS 3516: Computer Networks Prof. Yanhua Li Time: 9:00am 9:50am M, T, R, and F Location: AK219 Fall 2018 A-term 1 Some slides are originally from the course materials of the textbook Computer Networking: A Top Down Approach, 7th edition, by Jim Kurose, Keith Ross, Addison-Wesley March Copyright J.F Kurose and K.W. Ross, All Rights Reserved.

2 Chapter 1: roadmap 1.1 what is the Internet? nuts and bolts view service view 1.2 network edge end systems, access networks, links 1.3 network core packet switching, circuit switching, network structure Introduction 1-2

3 A closer look at network structure: network edge: hosts: clients and servers servers often in data centers v access networks, physical media: wired, wireless communication links mobile network home network global ISP regional ISP v network core: interconnected routers network of networks institutional network Introduction 1-3

4 Access networks and physical media Q: How to connect end systems to edge router? residential access nets institutional access networks (school, company) mobile access networks mobile network home network institutional network Introduction 1-4

5 Access net: home network wireless devices often combined in single box to/from headend or central office cable or DSL modem wireless access point (54 Mbps) router, firewall, NAT wired Ethernet (100 Mbps) Introduction 1-5

6 Chapter 1: roadmap 1.1 what is the Internet? nuts and bolts view service view 1.2 network edge end systems, access networks, links 1.3 network core packet switching, circuit switching, network structure Introduction 1-6

7 The network core mesh of interconnected routers with three key aspects in network core Link: Switching, Resource allocation (chp 1.3) Node: Routing & Forwarding (to be discussed in Network layer chp 4) Network: Network Core Structure / Management / Coordination (chp 1.3) Introduction 1-7

8 Packet-switching: store-and-forward L bits per packet source R bps R bps des+na+on takes L/R seconds to transmit (push out) L-bit packet into link at R bps store and forward: entire packet must arrive at router before it can be transmitted on next link end-end delay = 2L/R (assuming zero propagation delay) one-hop numerical example: L = 7.5 Mbits R = 1.5 Mbps one-hop transmission delay = 5 sec more on delay shortly Introduction 1-8

9 Host: sends packets of data host sending function: takes application message breaks into smaller chunks, known as packets, of length L bits transmits packet into access network at transmission rate R link transmission rate, aka link capacity, aka link bandwidth 2 host 1 two packets, L bits each R: link transmission rate packet transmission delay time needed to transmit L-bit packet into link = = L (bits) R (bits/sec) Introduction 1-9

10 Packet Switching: queueing delay, loss A R = 100 Mb/s C B queue of packets waiting for output link R = 1.5 Mb/s D E queuing and loss: if arrival rate (in bits) to link exceeds transmission rate of link for a period of time: packets will queue, wait to be transmitted on link packets can be dropped (lost) if memory (buffer) fills up Introduction 1-10

11 Two key network-core functions routing: determines sourcedestination route taken by packets routing algorithms forwarding: move packets from router s input to appropriate router output routing algorithm local forwarding table header value output link destination address in arriving packet s header Introduction1-11

12 Alternative core: circuit switching end-end resources allocated to, reserved for call between source & dest: in diagram, each link has four circuits. call gets 2 nd circuit in top link and 1 st circuit in right link. dedicated resources: no sharing circuit-like (guaranteed) performance circuit segment idle if not used by call (no sharing) commonly used in traditional telephone networks Introduction 1-12

13 Circuit switching: FDM versus TDM FDM Example: 4 users frequency TDM Dedicated bandwidth time frequency Dedicated time slots time Introduction 1-13

14 Analogy to Road Networks End systems=buildings Packet switches=intersections Links=road segments

15 Packet switching versus circuit switching is packet switching a slam dunk winner? Pros: great for bursty data (advantages) resource sharing simpler, no call setup Cons: excessive congestion possible: packet delay and loss protocols needed for reliable data transfer, congestion control Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? Like parking lots. 1-15

16 Packet switching versus circuit switching packet switching allows more users to use network! example: 1 Mb/s link each user: 100 kb/s when active active 10% of time.. N users 1 Mbps link circuit-switching: 10 users packet switching: with 35 users, probability > 10 active at same time is less small Q: probability of u 1, u 2,, u 10 are active, and u 11 -u 35 inactive? (1/10) 10 *(9/10) 25 * Check out the online interactive exercises for more examples Introduction 1-16

17 The network core Three key aspects in network core Link: Switching, Resource allocation (chp 1.3) Node: Routing & Forwarding (to be discussed in Network layer chp 4) Network: Network Core Structure / Management / Coordination (chp 1.3) Introduction 1-17

18 Lab-assignment 1 lab1.html Introduction 1-18

19 packet analyzer packet capture (pcap) copy of all Ethernet frames sent/ received application (www browser, client) application OS Transport (TCP/UDP) Network (IP) Link (Ethernet) Physical